Filter for 6 ghz wi-fi using transversely-excited film bulk acoustic resonators

ABSTRACT

A 6 GHz Wi-Fi bandpass filter includes a ladder filter circuit with two or more shunt transversely-excited film bulk acoustic resonators (XBARs) and two or more series XBARs. Each of the two or more shunt XBARS includes a diaphragm having an LN-equivalent thickness greater than or equal to 310 nm, and each of the two or more series XBARS includes a diaphragm having an LN-equivalent thickness less than or equal to 305 nm.

RELATED APPLICATION INFORMATION

This patent claims priority from provisional patent application63/168,093, titled XBAR FILTERS FOR WI-FI 6E, filed Mar. 30, 2021.

NOTICE OF COPYRIGHTS AND TRADE DRESS

A portion of the disclosure of this patent document contains materialwhich is subject to copyright protection. This patent document may showand/or describe matter which is or may become trade dress of the owner.The copyright and trade dress owner has no objection to the facsimilereproduction by anyone of the patent disclosure as it appears in thePatent and Trademark Office patent files or records, but otherwisereserves all copyright and trade dress rights whatsoever.

BACKGROUND Field

This disclosure relates to radio frequency filters using acoustic waveresonators, and specifically to filters for use in communicationsequipment.

Description of the Related Art

A radio frequency (RF) filter is a two-port device configured to passsome frequencies and to stop other frequencies, where “pass” meanstransmit with relatively low signal loss and “stop” means block orsubstantially attenuate. The range of frequencies passed by a filter isreferred to as the “pass-band” of the filter. The range of frequenciesstopped by such a filter is referred to as the “stop-band” of thefilter. A typical RF filter has at least one pass-band and at least onestop-band. Specific requirements on a pass-band or stop-band depend onthe specific application. For example, a “pass-band” may be defined as afrequency range where the insertion loss of a filter is better than adefined value such as 1 dB, 2 dB, or 3 dB. A “stop-band” may be definedas a frequency range where the rejection of a filter is greater than adefined value such as 20 dB, 30 dB, 40 dB, or greater depending onapplication.

RF filters are used in communications systems where information istransmitted over wireless links. For example, RF filters may be found inthe RF front-ends of cellular base stations, mobile telephone andcomputing devices, satellite transceivers and ground stations, IoT(Internet of Things) devices, laptop computers and tablets, fixed pointradio links, and other communications systems. RF filters are also usedin radar and electronic and information warfare systems.

RF filters typically require many design trade-offs to achieve, for eachspecific application, the best compromise between performance parameterssuch as insertion loss, rejection, isolation, power handling, linearity,size and cost. Specific design and manufacturing methods andenhancements can benefit simultaneously one or several of theserequirements.

Performance enhancements to the RF filters in a wireless system can havebroad impact to system performance. Improvements in RF filters can beleveraged to provide system performance improvements such as larger cellsize, longer battery life, higher data rates, greater network capacity,lower cost, enhanced security, higher reliability, etc. Theseimprovements can be realized at many levels of the wireless system bothseparately and in combination, for example at the RF module, RFtransceiver, mobile or fixed sub-system, or network levels.

High performance RF filters for present communication systems commonlyincorporate acoustic wave resonators including surface acoustic wave(SAW) resonators, bulk acoustic wave (BAW) resonators, film bulkacoustic wave resonators (FBAR), and other types of acoustic resonators.However, these existing technologies are not well-suited for use at thehigher frequencies and bandwidths proposed for future communicationsnetworks.

The desire for wider communication channel bandwidths will inevitablylead to the use of higher frequency communications bands. Radio accesstechnology for mobile telephone networks has been standardized by the3GPP (3^(rd) Generation Partnership Project). Radio access technologyfor 5^(th) generation mobile networks is defined in the 5G NR (newradio) standard. The 5G NR standard defines several new communicationsbands. Two of these new communications bands are n77, which uses thefrequency range from 3300 MHz to 4200 MHz, and n79, which uses thefrequency range from 4400 MHz to 5000 MHz. Both band n77 and band n79use time-division duplexing (TDD), such that a communications deviceoperating in band n77 and/or band n79 uses the same frequencies for bothuplink and downlink transmissions. Bandpass filters for bands n77 andn79 must be capable of handling the transmit power of the communicationsdevice. Wi-Fi™ bands at 5 GHz and 6 GHz also require high frequency andwide bandwidth. The 5G NR standard also defines millimeter wavecommunication bands with frequencies between 24.25 GHz and 40 GHz.

The Transversely-Excited Film Bulk Acoustic Resonator (XBAR) is anacoustic resonator structure for use in microwave filters. The XBAR isdescribed in U.S. Pat. No. 10,491,291, titled TRANSVERSELY EXCITED FILMBULK ACOUSTIC RESONATOR. An XBAR resonator comprises an interdigitaltransducer (IDT) at least partially disposed on a thin floating layer,or diaphragm which is or includes a layer of a single-crystalpiezoelectric material. The IDT includes a first set of parallelfingers, extending from a first busbar and a second set of parallelfingers extending from a second busbar. The first and second sets ofparallel fingers are interleaved. A microwave signal applied to the IDTexcites a shear primary acoustic wave in the piezoelectric diaphragm.XBAR resonators provide very high electromechanical coupling and highfrequency capability. XBAR resonators may be used in a variety of RFfilters including band-reject filters, band-pass filters, duplexers, andmultiplexers. XBARs are well suited for use in filters forcommunications bands with frequencies above 3 GHz.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of an exemplary band-pass filter.

FIG. 2 has a schematic plan view and two schematic cross-sectional viewsof a transversely-excited film bulk acoustic resonator (XBAR).

FIG. 3 is an expanded schematic cross-sectional view of a portion of theXBAR of FIG. 2.

FIG. 4 is a graphic illustrating a shear primary acoustic mode in anXBAR.

FIG. 5 is a graph of the resonance frequency of an XBAR as a function ofpiezoelectric plate thickness for four different dielectric thicknesses.

FIG. 6 is a graph of the resonance frequency of an XBAR as a function ofequivalent diaphragm thickness for four different dielectricthicknesses.

FIG. 7 is a graph of the anti-resonance frequency of an XBAR as afunction of piezoelectric plate thickness for four different dielectricthicknesses.

FIG. 8 is a graph of the anti-resonance frequency of an XBAR as afunction of equivalent diaphragm thickness for four different dielectricthicknesses.

FIG. 9 is a schematic diagram of an exemplary band-pass filter for 6 GHzWi-Fi.

FIG. 10 is a graph of the magnitude of the input/output transferfunction S21 for the exemplary band-pass filter of FIG. 9.

FIG. 11 is a flow chart of a process for fabricating an XBAR or a filterincorporating XBARs.

Throughout this description, elements appearing in figures are assignedthree-digit or four-digit reference designators, where the two leastsignificant digits are specific to the element and the one or two mostsignificant digit is the figure number where the element is firstintroduced. An element that is not described in conjunction with afigure may be presumed to have the same characteristics and function asa previously-described element having the same reference designator.

DETAILED DESCRIPTION

Description of Apparatus

FIG. 1 is a schematic circuit diagram of an exemplary band-pass filter100 using five XBARs X1-X5. The filter 100 may be, for example, aband-pass filter for use in a communication device. The filter 100 has aconventional ladder filter architecture including three seriesresonators X1, X3, X5 and two shunt resonators X2, X4. The three seriesresonators X1, X3, X5 are connected in series between a first port P1and a second port P2. The filter 100 is bidirectional and either portmay serve as the input or output of the filter. The two shunt resonatorsX2, X4 are connected from nodes between the series resonators to ground.All the shunt resonators and series resonators may be XBARs.

Each of the resonators X1 to X5 has a resonance frequency and ananti-resonance frequency. In over-simplified terms, each resonator iseffectively a short circuit at its resonance frequency and effectivelyan open circuit at its anti-resonance frequency. Each resonator X1 to X5creates a “transmission zero”, where the transmission between the in andout ports is very low. Note that the transmission at a “transmissionzero” is not actually zero due to energy leakage through parasiticcomponents and other effects. The three series resonators X1, X3, X5create transmission zeros at their respective anti-resonance frequencies(where each resonator is effectively an open circuit). The two shuntresonators X2, X4 create transmission zeros at their respectiveresonance frequencies (where each resonator is effectively a shortcircuit). In a typical band-pass filter using acoustic resonators,resonance frequencies of the shunt resonators are positioned below thepassband of the filter and the anti-resonance frequencies of the shuntresonators are within the passband. Resonance frequencies of the seriesresonators are within the passband and the anti-resonance frequencies ofthe series resonators are positioned above the passband.

Referring now to FIG. 2, the structure of XBARs will be described infurther detail. FIG. 2 shows a simplified schematic top view andorthogonal cross-sectional views of an XBAR 200. The XBAR 200 is made upof a thin film conductor pattern formed on a surface of a piezoelectricplate 210 having a front surface 212 and a back surface 214. The frontand back surfaces are essentially parallel. “Essentially parallel” meansparallel to the extent possible with normal manufacturing tolerances.The piezoelectric plate is a thin single-crystal layer of apiezoelectric material such as lithium niobate, lithium tantalate,lanthanum gallium silicate, gallium nitride, or aluminum nitride. Thepiezoelectric plate is cut such that the orientation of the X, Y, and Zcrystalline axes with respect to the front and back surfaces is knownand consistent. In the examples presented in this patent, thepiezoelectric plates are rotated YX-cut. However, XBARs may befabricated on piezoelectric plates with other crystallographicorientations including rotated Z-cut and rotated Z-cut.

The back surface 214 of the piezoelectric plate 210 is attached to asurface 222 of the substrate 220 except for a portion of thepiezoelectric plate 210 that forms a diaphragm 215 spanning a cavity 240formed in the substrate 220. The cavity 240 has a perimeter defined bythe intersection of the cavity and the surface 222 of the substrate 220.The portion of the piezoelectric plate that spans the cavity is referredto herein as the “diaphragm” due to its physical resemblance to thediaphragm of a microphone. As shown in FIG. 2, the diaphragm 215 iscontiguous with the rest of the piezoelectric plate 210 around all ofthe perimeter 245 of the cavity 240. In this context, “contiguous” means“continuously connected without any intervening item”.

The substrate 220 provides mechanical support to the piezoelectric plate210. The substrate 220 may be, for example, silicon, sapphire, quartz,or some other material or combination of materials. The back surface 214of the piezoelectric plate 210 may be attached to the substrate 220using a wafer bonding process. Alternatively, the piezoelectric plate210 may be grown on the substrate 220 or otherwise attached to thesubstrate. The piezoelectric plate 210 may be attached directly to thesubstrate or may be attached to the substrate 220 via one or moreintermediate material layers.

The cavity 240 is an empty space within a solid body of the resonator200. The cavity 240 may be a hole completely through the substrate 220(as shown in Section A-A and Section B-B) or a recess in the substrate220 (not shown). The cavity 240 may be formed, for example, by selectiveetching of the substrate 220 before or after the piezoelectric plate 210and the substrate 220 are attached.

The conductor pattern of the XBAR 200 includes an interdigitaltransducer (IDT) 230. An IDT is an electrode structure for convertingbetween electrical and acoustic energy in piezoelectric devices. The IDT230 includes a first plurality of parallel elongated conductors,commonly called “fingers”, such as finger 236, extending from a firstbusbar 232. The IDT 230 includes a second plurality of fingers extendingfrom a second busbar 234. The first and second pluralities of parallelfingers are interleaved. The interleaved fingers overlap for a distanceAP, commonly referred to as the “aperture” of the IDT. Thecenter-to-center distance L between the outermost fingers of the IDT 230is the “length” of the IDT.

The term “busbar” refers to the conductors that interconnect the firstand second sets of fingers in an IDT. As shown in FIG. 2, each busbar232, 234 is an elongated rectangular conductor with a long axisorthogonal to the interleaved fingers and having a length approximatelyequal to the length L of the IDT. The busbars of an IDT need not berectangular or orthogonal to the interleaved fingers and may havelengths longer than the length of the IDT.

The first and second busbars 232, 234 serve as the terminals of the XBAR200. A radio frequency or microwave signal applied between the twobusbars 232, 234 of the IDT 230 excites a primary acoustic mode withinthe piezoelectric plate 210. As will be discussed in further detail, theprimary acoustic mode is a bulk shear mode where acoustic energypropagates along a direction substantially orthogonal to the surface ofthe piezoelectric plate 210, which is also normal, or transverse, to thedirection of the electric field created by the IDT fingers. Thus, theXBAR is considered a transversely-excited film bulk wave resonator.

The IDT 230 is positioned on the piezoelectric plate 210 such that atleast a substantial portion of the fingers of the IDT 230 are disposedon the diaphragm 215 of the piezoelectric plate that spans, or issuspended over, the cavity 240. As shown in FIG. 2, the cavity 240 has arectangular shape with an extent greater than the aperture AP and lengthL of the IDT 230. A cavity of an XBAR may have a different shape, suchas a regular or irregular polygon. The cavity of an XBAR may have moreor fewer than four sides, which may be straight or curved.

For ease of presentation in FIG. 2, the geometric pitch and width of theIDT fingers is greatly exaggerated with respect to the length (dimensionL) and aperture (dimension AP) of the XBAR. An XBAR for a 5G device willhave more than ten parallel fingers in the IDT 210. An XBAR may havehundreds, possibly thousands, of parallel fingers in the IDT 210.Similarly, the thickness of the fingers in the cross-sectional views isgreatly exaggerated in the drawings.

FIG. 3 shows a detailed schematic cross-sectional view of the XBAR 200.The piezoelectric plate 210 is a single-crystal layer of piezoelectricmaterial having a thickness ts. ts may be, for example, 100 nm to 1500nm. When used in filters for 5G NR and WiFi bands from 3.3 GHZ to 7 GHz,the thickness ts may be, for example, 250 nm to 700 nm.

A front-side dielectric layer 314 may be formed on the front side of thepiezoelectric plate 210. The “front side” of the XBAR is the surfacefacing away from the substrate. The front-side dielectric layer 314 hasa thickness tfd. The front-side dielectric layer 314 may be formed onlybetween the IDT fingers (see IDT finger 338 a). The front sidedielectric layer 314 may also be deposited over the IDT fingers (see IDTfinger 338 b). A back-side dielectric layer 316 may be formed on theback side of the piezoelectric plate 210. The back-side dielectric layer316 has a thickness tbd. The front-side and back-side dielectric layers314, 316 may be a non-piezoelectric dielectric material, such as silicondioxide or silicon nitride. tfd and tbd may be, for example, 0 to 500nm. tfd and tbd are each typically less than one-half of the thicknessis of the piezoelectric plate. tfd and tbd are not necessarily equal,and the front-side and back-side dielectric layers 314, 316 are notnecessarily the same material. Either or both of the front-side andback-side dielectric layers 314, 316 may be formed of multiple layers oftwo or more materials.

The IDT fingers 338 a, 338 b may be one or more layers of aluminum, asubstantially aluminum alloy, copper, a substantially copper alloy,beryllium, gold, molybdenum, or some other conductive material. Thin(relative to the total thickness of the conductors) layers of othermetals, such as chromium or titanium, may be formed under and/or overthe fingers to improve adhesion between the fingers and thepiezoelectric plate 210 and/or to passivate or encapsulate the fingers.The busbars (232, 234 in FIG. 2) of the IDT may be made of the same ordifferent materials as the fingers. As shown in FIG. 3, the IDT fingers338 a, 338 b have rectangular or trapezoidal cross-sections. The IDTfingers may have some other cross-sectional shape.

Dimension p is the center-to-center spacing or “pitch” of the IDTfingers, which may be referred to as the pitch of the IDT and/or thepitch of the XBAR. Dimension w is the width or “mark” of the IDTfingers. The IDT of an XBAR differs substantially from the IDTs used insurface acoustic wave (SAW) resonators. In a SAW resonator, the pitch ofthe IDT is one-half of the acoustic wavelength at the resonancefrequency. Additionally, the mark-to-pitch ratio of a SAW resonator IDTis typically close to 0.5 (i.e., the mark or finger width is aboutone-fourth of the acoustic wavelength at resonance). In an XBAR, thepitch p of the IDT is typically 2 to 20 times the width w of thefingers. In addition, the pitch p of the IDT is typically 2 to 20 timesthe thickness tp of the piezoelectric plate 210. The width of the IDTfingers in an XBAR is not constrained to one-fourth of the acousticwavelength at resonance. For example, the width of XBAR IDT fingers maybe 500 nm or greater, such that the IDT can be fabricated using opticallithography. The thickness tm of the IDT fingers may be from 100 nm toabout equal to the width w. The thickness of the busbars (232, 234 inFIG. 2) of the IDT may be the same as, or greater than, the thickness tmof the IDT fingers.

FIG. 4 is a graphical illustration of the primary acoustic mode ofinterest in an XBAR. FIG. 4 shows a small portion of an XBAR 400including a piezoelectric plate 410 and three interleaved IDT fingers430. A radio frequency (RF) voltage is applied to the interleavedfingers 430. This voltage creates a time-varying electric field betweenthe fingers. The direction of the electric field is primarily lateral,or parallel to the surface of the piezoelectric plate 410, as indicatedby the arrows labeled “electric field”. Since the dielectric constant ofthe piezoelectric plate is significantly higher than the surroundingair, the electric field is highly concentrated in the plate relative tothe air. The lateral electric field introduces shear deformation, andthus strongly excites a shear-mode acoustic mode, in the piezoelectricplate 410. Shear deformation is deformation in which parallel planes ina material remain parallel and maintain a constant distance whiletranslating relative to each other. A “shear acoustic mode” is anacoustic vibration mode in a medium that results in shear deformation ofthe medium. The shear deformations in the XBAR 400 are represented bythe curves 460, with the adjacent small arrows providing a schematicindication of the direction and magnitude of atomic motion. The degreeof atomic motion, as well as the thickness of the piezoelectric plate410, have been greatly exaggerated for ease of visualization. While theatomic motions are predominantly lateral (i.e., horizontal as shown inFIG. 4), the direction of acoustic energy flow of the excited primaryshear acoustic mode is substantially orthogonal to the surface of thepiezoelectric plate, as indicated by the arrow 465.

An acoustic resonator based on shear acoustic wave resonances canachieve better performance than current state-of-the artfilm-bulk-acoustic-resonators (FBAR) and solidly-mounted-resonatorbulk-acoustic-wave (SMR BAW) devices where the electric field is appliedin the thickness direction. In such devices, the acoustic mode iscompressive with atomic motions and the direction of acoustic energyflow in the thickness direction. In addition, the piezoelectric couplingfor shear wave XBAR resonances can be high (>20%) compared to otheracoustic resonators. High piezoelectric coupling enables the design andimplementation of microwave and millimeter-wave filters with appreciablebandwidth.

The resonance frequency of an XBAR is determined by the thickness of thediaphragm and the pitch and mark (width) of the IDT fingers. Thethickness of the diaphragm, which includes the thickness of thepiezoelectric plate and the thicknesses of front-side and/or back-sidedielectric layers, is the dominant factor determining the resonancefrequency. The tuning range provided by varying the pitch and/or mark islimited to only a few percent. For broad bandwidth filters such asband-pass filter for 5G NR bands n77 and n79, 5 GHz Wi-Fi and 6 GHzWi-Fi, the tuning range provided by varying the IDT pitch and fingerwidth is insufficient to provide the necessary separation between theresonance frequencies of the shunt and the anti-resonance frequencies ofthe series resonators.

FIG. 5 is graph 500 of the relationship between the resonance frequency,piezoelectric plate thickness, and top-side dielectric thickness forrepresentative XBARs using lithium niobate piezoelectric plates withEuler angles of [0°, β, 0° ], where 0°<β≤60°, as described in U.S. Pat.No. 10,790,802. For historical reasons, such plates are commonlyreferred to as “rotated YX-cut” where the “rotation angle” is β+90°.

The solid line 510 is a plot of the dependence of resonance frequency onpiezoelectric plate thickness without a front-side. The short-dash line520 is a plot of the dependence of resonance frequency on piezoelectricplate thickness with a front-side dielectric layer thickness of 30 nm.The dot-dash line 530 is a plot of the dependence of resonance frequencyon piezoelectric plate thickness with a front-side dielectric layerthickness of 60 nm. The long-dash line 540 is a plot of the dependenceof resonance frequency on piezoelectric plate thickness with afront-side dielectric layer thickness of 90 nm. In all cases, thepiezoelectric plate is 128-degree YX-cut lithium niobate, the IDT pitchis 10 times the piezoelectric plate thickness, the mark/pitch ratio is0.25, and the IDT electrodes are aluminum with a thickness of 1.25 timesthe piezoelectric plate thickness. The dielectric layers are SiO₂. Thereis no back-side dielectric layer.

The dash-dot-dot line 550 marks the lower edge of the 6 GHz Wi-Fi bandsat 5.935 GHz. The dash-dot-dot line 555 marks the upper edge of the 6GHz Wi-Fi bands at 7.12 GHz. The solid dot 560 represents the shuntresonators of an exemplary filter that will be described in furtherdetail subsequently. The solid dot 565 represents the series resonatorsof the exemplary filter. Note that the resonance frequencies of theshunt resonator (solid dot 550) are several hundred MHz below the loweredge of the 6 GHz Wi-Fi bands and the resonance frequencies of theseries resonators are between the upper and lower edges (i.e., withinthe passband of the exemplary filter).

From FIG. 5 it can also be seen that, at 6 GHz, a 90 mm layer of SiO₂has the same effect on resonance frequency as roughly a 55 nm change inthe thickness of the lithium niobate piezoelectric plate. Thethicknesses of the lithium niobate piezoelectric plate and thefront-side SiO₂ layer can be combined to provide an equivalent thicknessof the XBAR diaphragm as follows:

teqr=tp+kr(tfsd)  (1)

where teqr is the “LN-equivalent” thickness (i.e., the thickness oflithium niobate having the same resonance frequency) of the diaphragmfor shunt resonators. tp and tfsd are the thickness of the piezoelectricplate and front-side dielectric layer as previously defined, and kr isproportionally constant for shunt resonators. kr depends on the materialof the front-side dielectric layer. When the front-side dielectric layeris SiO₂, kr is approximately 0.57.

FIG. 6 is a graph 600 of the dependence of resonance frequency onLN-equivalent diaphragm thickness. The data shown in FIG. 6 is the sameas the data shown in FIG. 5, with LN-equivalent thickness calculatedusing formula (1) with k=0.57. The composite line 610 is made up of theresonance frequency data points for four different dielectric (oxide)thicknesses. These data points form a reasonably continuous curve.

The dash-dot-dot lines 650 marks the lower edge of the 6 GHz Wi-Fi bandsat 5.935 GHz. The dash-dot-dot line 655 marks the upper edge of the 6GHz Wi-Fi bands at 7.12 GHz. The solid dot 660 represents the shuntresonators of the exemplary filter. The solid dot 665 represents theseries resonators of the exemplary filter.

The data in FIG. 6 is specific to XBARs with the IDT pitch equal to 10times the piezoelectric plate thickness. A preferred range of the pitchof an XBAR is 6 to 12.5 times the IDT pitch. Electromechanical couplingdecreases sharply for pitch less than 6 time the piezoelectric platethickness, reducing the difference between the resonance andanti-resonance frequencies. Increasing the pitch above 12.5 times thepiezoelectric plate thickness reduces capacitance per unit resonatorarea with little benefit in terms of electromechanical coupling orfrequency change.

Reducing the pitch to 6 times the piezoelectric plate thicknessincreases resonance frequency by about 4.5% compared to an XBAR withpitch equal to 10 times the piezoelectric plate thickness. Increasingthe pitch to 12.5 times the piezoelectric plate thickness reducesresonance frequency by about 1.1% compared to an XBAR with pitch equalto 10 times the piezoelectric plate thickness. Conversely, an XBAR withpitch equal to 6 times the piezoelectric plate thickness will need a4.5% thicker diaphragm to have the same resonance frequency as an XBARwith pitch equal to 10 times the piezoelectric plate thickness. An XBARwith pitch equal to 12.5 times the piezoelectric plate thickness willneed a 1.1% thinner diaphragm to have the same resonance frequency as anXBAR with pitch equal to 10 times the piezoelectric plate thickness.

FIG. 6 shows that, in order for the resonance frequencies of shuntresonators to be below the lower edge of the passband (line 650), theLN-equivalent diaphragm thickness of shunt resonators must be greaterthan 310 nm. The actual LN-equivalent diaphragm thickness of shuntresonators is determined by multiple factors including the requiredband-edge sharpness, the Q-factor of the shunt resonators, requiredoperating temperature range, and allowance for manufacturing tolerances.The LN-equivalent diaphragm thickness may be 320 nm to 340 nm.

The LN-equivalent thickness of the diaphragm of the shunt resonatorsneed not be the same. One or more additional dielectric layer may beformed over a subset of the shunt resonators to further lower theirresonance frequencies (for example to increase attenuation in a stopband below the lower band edge). While there is no absolute upper limiton the thickness of a front-side dielectric layer, XBARs withtfsd/tp>0.30 tend to have substantial spurious modes.

The anti-resonance frequency of an XBAR is determined by the resonancefrequency (which depends on the thickness of the diaphragm and the pitchand width of the IDT fingers) and the electromechanical coupling of theresonator. Electromechanical coupling is primarily determined by the cutangle of the piezoelectric material, the thickness of dielectric layersif present, and the pitch of the IDT. FIG. 7 is a graph 700 of therelationship between the anti-resonance frequency, piezoelectric platethickness, and top-side dielectric thickness for representative XBARs.In all cases, the piezoelectric plate is 128-degree YX-cut lithiumniobate, the IDT pitch is 10 times the piezoelectric plate thickness,the mark/pitch ratio is 0.25, and the IDT electrodes are aluminum with athickness of 1.25 times the piezoelectric plate thickness. Thedielectric layers are SiO₂. There is no back-side dielectric layer.

The solid line 710 is a plot of the dependence of anti-resonancefrequency on piezoelectric plate thickness without a front-sidedielectric layer. The short-dash line 720 is a plot of the dependence ofanti-resonance frequency on piezoelectric plate thickness with afront-side dielectric layer thickness of 30 nm. The dot-dash line 730 isa plot of the dependence of anti-resonance frequency on piezoelectricplate thickness with a front-side dielectric layer thickness of 60 nm.The long-dash line 740 is a plot of the dependence of anti-resonancefrequency on piezoelectric plate thickness with a front-side dielectriclayer thickness of 90 nm.

The dash-dot-dot lines 750 marks the lower edge of the 6 GHz Wi-Fi bandsat 5.935 GHz. The dash-dot-dot line 755 marks the upper edge of the 6GHz Wi-Fi bands at 7.12 GHz. The solid dot 760 represents the shuntresonators of the exemplary filter. The solid dot 765 represents theseries resonators of the exemplary filter.

Reducing the pitch to 6 times the piezoelectric plate thicknessincreases anti-resonance frequency by about 3.2% compared to an XBARwith pitch equal to 10 times the piezoelectric plate thickness.Increasing the pitch to 12.5 times the piezoelectric plate thicknessreduces anti-resonance frequency by about 1.0% compared to an XBAR withpitch equal to 10 times the piezoelectric plate thickness. An XBAR withpitch equal to 6 times the piezoelectric plate thickness will need a3.2% thicker diaphragm to have the same anti-resonance frequency as anXBAR with pitch equal to 10 times the piezoelectric plate thickness.

As previously described, in a ladder filter circuit, series resonatorsprovide transmission zeros at frequencies above the upper edge of thefilter passband. To this end, the anti-resonance frequencies of theseries resonators of a 6 GHz Wi-Fi filter must be greater than 5120 MHz.Exactly how much greater than 5120 MHz depends on the filterspecifications, the Q-factors of the series resonators, and allowancesfor manufacturing tolerances and temperature variations includingtemperature increases due to power consumed in the filter duringtransmission.

The thicknesses of the lithium niobate piezoelectric plate and thefront-side SiO₂ layer can be combined to provide an equivalent thicknessof the XBAR diaphragm as follows:

teqa=tp+ka(tfsd)  (1)

where teqa is the “LN-equivalent” thickness (i.e., the thickness oflithium niobate having the same anti-resonance frequency) of thediaphragm for series resonators. tp and tfsd are the thickness of thepiezoelectric plate and front-side dielectric layer as previouslydefined, and ka is proportionally constant for series resonators. kadepends on the material of the front-side dielectric layer. When thefront-side dielectric layer is SiO₂, ka is approximately 0.45.

FIG. 8 is a graph 800 of the dependence of anti-resonance frequency onLN-equivalent diaphragm thickness. The data shown in FIG. 8 is the sameas the data shown in FIG. 7, with LN-equivalent thickness calculatedusing formula (1) with ka=0.45. The composite line 810 is made up of theanti-resonance frequency data points for four different dielectricthicknesses. These data points form a reasonably continuous curve,except for the combination of 90 nm front-side dielectric on relativelythin piezoelectric substrates.

The dash-dot-dot lines 750 marks the lower edge of the 6 GHz Wi-Fi bandsat 5.935 GHz. The dash-dot-dot line 755 marks the upper edge of the 6GHz Wi-Fi bands at 7.12 GHz. The solid dot 760 represents the shuntresonators of the exemplary filter. The solid dot 765 represents theseries resonators of the exemplary filter.

FIG. 8 shows that, in order for the anti-resonance frequencies of seriesresonators to be above the upper lower edge of the passband (line 855),the LN-equivalent diaphragm thickness of series resonators must be lessthan 305 nm. The actual LN-equivalent diaphragm thickness of seriesresonators is determined by multiple factors including the requiredband-edge sharpness, the Q-factor of the series resonators, requiredoperating temperature range, and allowance for manufacturing tolerances.The LN-equivalent diaphragm thickness may typically be less than 295 nm.

The LN-equivalent thickness of the diaphragm of the series resonatorsneed not be the same. One or more additional dielectric layer may beformed over a subset of the series resonators to further lower theirresonance frequencies (for example to increase attenuation in a stopband below the lower band edge).

FIG. 9 is a schematic circuit diagram of an exemplary band 6 GHz Wi-Fiband-pass filter 900 using four series resonators Se1, Se2, Se3, Se4 andfour shunt resonators Sh1, Sh2, Sh3, Sh4. The filter 900 has aconventional ladder filter architecture. The four series resonators Se1,Se2, Se3, Se4 are connected in series between a first port P1 and asecond port P2. The filter 900 is bidirectional and either port mayserve as the input or output of the filter. Shunt resonator Sh1 isconnected from port P1 to ground. The other three shunt resonators Sh2,Sh3, Sh4, are connected from nodes between the series resonators toground. All the shunt resonators and series resonators are composed ofmultiple XBAR sub-resonators. The number of sub-resonators is indicatedbelow the reference designator (e.g., “×4”) of each resonator.Resonators composed of four sub-resonators, such as resonator Se3, areconnected is an series/parallel combination as shown in the detailschematic diagram of sub-resonators Se3A, Se3B, Se3C, and Se3D.Resonators composed of two sub-resonators, such as resonator Sh3, havethe two sub-resonators connected in parallel as shown in the detailschematic diagram of sub-resonators Sh3A and Sh3B. Dividing an XBAR intomultiple sub-resonators has a primary benefit of reducing the peakstress that would occur if each XBAR had a single large diaphragm. Themultiple sub-resonators of each resonator typically, but notnecessarily, have the same aperture and approximately the same length.

The pitch and mark of the sub-resonators comprising any of the series orshunt resonators are not necessarily the same. As shown in the detailedschematic diagram, series resonator Se3 is composed of foursub-resonators Se3A, Se3B, Se3C, Se3D. Each of the sub-resonatorsSe3A-Se3D may have a unique mark and pitch, which is to say the mark andpitch of any sub-resonator is different from the mark and pitch of eachother sub-resonator. Further, the pitch and mark of each sub-resonatoris not necessarily constant over the length of the IDT of thesub-resonator.

Small variations (e.g., ±1%) in pitch within the IDT of an XBAR mayreduce the amplitude of spurious modes. The small variations in pitchshift the frequencies of spurious modes such that the spurious modes donot add constructively over the area of the XBAR. These small pitchvariations have a negligible effect on the resonance and anti-resonancefrequencies. The small changes in pitch between the sub-resonators Se3A,Se3B, Se3C, Se3D have a similar effect. The spurious modes of thesub-resonators do not add, resulting in lower overall spurious modeamplitude. All the series resonators Se1, Se2, Se3, Se4 may have the IDTpitch and/or mark varied between sub-resonators and/or withinsub-resonators.

The filter 900 uses a dielectric frequency setting layer, represented bythe dashed rectangle 920, to offset the resonance frequencies of theshunt resonators from the resonance frequencies of the seriesresonators. The series Se1-Se4 have a thin dielectric layer or nodielectric layer and the shunt resonators Sh1-Sh4 include a thickerdielectric layer. The difference in dielectric layer thickness lowersthe resonance frequencies of the shunt resonators with respect to theresonance frequencies of the series resonators.

The filter 900 is exemplary, and other filters designs using more orfewer resonators are possible. A 6 GHz Wi-Fi bandpass filter willinclude at least two series resonators and at least two shuntresonators.

FIG. 10 is a graph of the performance of an exemplary 6 GHz Wi-Fibandpass filter, which may be, or be similar to, the filter 900 of FIG.9. Specifically, the curve 1010 is a plot of the magnitude of theinput/output transfer function S21 of the exemplary filter over afrequency range from 5 GHz to 8.5 GHz. The passband of the exemplaryfilter encompasses the 6 GHz Wi-Fi bands from 5.935 GHz to 7.12 GHz.

The physical characteristics of the exemplary filter are as follows:

-   -   piezoelectric plate: 128° YX cut lithium niobate;

piezoelectric plate thickness: 276 nm;

-   -   front side dielectric: SiO₂;

frontside dielectric thickness: 0 (series resonators);

frontside dielectric thickness: 86 nm (shunt resonators);

LN-equivalent diaphragm thickness: 276 nm (series resonators);

LN-equivalent diaphragm thickness: 325 nm (shunt resonators);

-   -   IDT fingers: 360 nm, substantially aluminum;

IDT pitch/piezoelectric plate thickness: 10.6-11.1;

-   -   IDT finger width/pitch: 0.18-0.24.

Description of Methods

FIG. 12 is a simplified flow chart showing a process 1200 for making awafer with a plurality of chips containing XBARs. The process 1200starts at 1205 with a device substrate and a plate of piezoelectricmaterial attached to a sacrificial substrate. The process 1200 ends at1295 with a plurality of completed XBAR chips. The flow chart of FIG. 12includes only major process steps. Various conventional process steps(e.g., surface preparation, cleaning, inspection, baking, annealing,monitoring, testing, etc.) may be performed before, between, after, andduring the steps shown in FIG. 12. With the exception of step 1280, allof the actions are performed concurrently on all chips on the wafer.

The flow chart of FIG. 12 captures three variations of the process 1200for making an XBAR which differ in when and how cavities are formed inthe device substrate. The cavities may be formed at steps 1210A, 1210B,or 1210C. Only one of these steps is performed in each of the threevariations of the process 1200.

The piezoelectric plate may be, for example, rotated YX-cut lithiumniobate. The piezoelectric plate may be some other material and/or someother cut. The substrate may preferably be silicon. The substrate may besome other material that allows formation of deep cavities by etching orother processing.

In one variation of the process 1200, one or more cavities are formed inthe device substrate at 1210A before the piezoelectric plate is bondedto the substrate at 1215. A separate cavity may be formed for eachresonator on the chip. The one or more cavities may be formed usingconventional photolithographic and etching techniques. Typically, thecavities formed at 1210A will not penetrate through the devicesubstrate.

At 1215, the piezoelectric plate is bonded to the device substrate. Thepiezoelectric plate and the substrate may be bonded by a wafer bondingprocess. Typically, the mating surfaces of the substrate and thepiezoelectric plate are highly polished. One or more layers ofintermediate materials, such as an oxide or metal, may be formed ordeposited on the mating surface of one or both of the piezoelectricplate and the substrate. One or both mating surfaces may be activatedusing, for example, a plasma process. The mating surfaces may then bepressed together with considerable force to establish molecular bondsbetween the piezoelectric plate and the substrate or intermediatematerial layers.

The sacrificial substrate may be removed at 1220, exposing a frontsurface of the piezoelectric plate. The actions at 1220 may includefurther processes, such as polishing and/or annealing, to prepare theexposed surface for subsequent process steps.

A conductor pattern, including IDTs of each XBAR, is formed at 1230 bydepositing and patterning one or more conductor layers on the frontsurface of the piezoelectric plate. The conductor layer may be, forexample, aluminum or an aluminum alloy with a thickness of 50 nm to 150nm. Optionally, one or more layers of other materials may be disposedbelow (i.e., between the conductor layer and the piezoelectric plate)and/or on top of the conductor layer. For example, a thin film oftitanium, chrome, or other metal may be used to improve the adhesionbetween the conductor layer and the piezoelectric plate. A conductionenhancement layer of gold, aluminum, copper or other higher conductivitymetal may be formed over portions of the conductor pattern (for examplethe IDT busbars and interconnections between the IDTs).

The conductor pattern may be formed at 1230 by depositing the conductorlayer and, optionally, one or more other metal layers in sequence overthe surface of the piezoelectric plate. The excess metal may then beremoved by etching through patterned photoresist. The conductor layercan be etched, for example, by plasma etching, reactive ion etching, wetchemical etching, and other etching techniques.

Alternatively, the conductor pattern may be formed at 1230 using alift-off process. Photoresist may be deposited over the piezoelectricplate. and patterned to define the conductor pattern. The conductorlayer and, optionally, one or more other layers may be deposited insequence over the surface of the piezoelectric plate. The photoresistmay then be removed, which removes the excess material, leaving theconductor pattern.

One or more optional frequency setting dielectric layers may be formedat 1240 by depositing and patterning one or more dielectric layers onthe front surface of the piezoelectric plate. The dielectric layer(s)may be formed between, and optionally over, the IDT fingers of some, butnot all XBARs. The frequency setting dielectric layer(s) are typicallySiO₂, but may be Si₃N₄, Al₂O₃, or some other dielectric material. Thethickness of each frequency setting dielectric layer is determined bythe desire frequency shift. In the example of FIG. 8 and FIG. 10, afrequency setting dielectric layer 40 nm thick is formed over the IDTsof XBARs SE1, SE2, and SE3.

A passivation/tuning dielectric layer is formed at 1250 by depositing adielectric material over all of the front surface of the piezoelectricplate except pads used for electric connections to circuitry external tothe chip. The passivation/tuning layer may be SiO₂, Si₃N₄, Al₂O₃, someother dielectric material, or a combination of two or more materials.The thickness of passivation/tuning layer is determined by the minimumamount of dielectric material required to deal the surface of the chipplus the amount of sacrificial material possibly needed for frequencytuning at 1270. In the example of FIG. 9 and FIG. 10, apassivation/tuning layer 20 nm thick (after tuning) is assumed over theIDTs of all XBARs.

In a second variation of the process 1200, one or more cavities areformed in the back side of the substrate at 1210B. A separate cavity maybe formed for each resonator on the chip. The one or more cavities maybe formed using an anisotropic or orientation-dependent dry or wet etchto open holes through the back side of the substrate to thepiezoelectric plate.

In a third variation of the process 1200, one or more cavities in theform of recesses in the substrate may be formed at 1210C by etching thesubstrate using an etchant introduced through openings in thepiezoelectric plate. A separate cavity may be formed for each resonatorin a filter device. The one or more cavities formed at 1210C will notpenetrate through the substrate.

In all variations of the process 1200, some or all of XBARs on each chipmay be measured or tested at 1260. For example, the admittance of someor all XBARs may be measured at RF frequencies to determine theresonance and/or the anti-resonance frequencies. The measuredfrequencies may be compared to the intended frequencies and a map offrequency errors over the surface of the wafer may be prepared.

At 1270, the frequency of some or all XBARs may be tuned by selectivelyremoving material from the surface of the passivation/tuning layer inaccordance with the frequency error map developed at 1260. The selectivematerial removal may be done, for example, using a scanning ion mill orother tool.

The chips may then be completed at 1280. Actions that may occur at 1280include forming bonding pads or solder bumps or other means for makingconnection between the chips and external circuitry, additional testing,and excising individual chips from the wafer containing multiple chips.After the chips are completed, the process 1200 ends at 1295.

FIG. 13 is a flow chart of a method 1300 for fabricating a split-ladderfilter device, which may be the split ladder filter device 900 of FIG.9. The method 1300 starts at 1310 and concludes at 1390 with a completedfilter device.

At 1320, a first chip is fabricated using the process of FIG. 12 with afirst piezoelectric plate thickness. The first chip contains one, some,or all of the series resonators of the filter device. The first chip maybe a portion of a first large multi-chip wafer such that multiple copiesof the first chip are produced during each repetition of the step 1320.In this case, individual chips may be excised from the wafer and testedas part of the action at 1320.

At 1330, a second chip is fabricated using the process of FIG. 12 with asecond piezoelectric plate thickness. The second chip contains one,some, or all of the shunt resonators of the filter device. The secondchip may be a portion of a second large multi-chip wafer such thatmultiple copies of the second chip are produced during each repetitionof the step 1330. In this case, individual chips may be excised from thewafer and tested as part of the action at 1330.

At 1340, a circuit card is fabricated. The circuit card may be, forexample, a printed wiring board or an LTCC card or some other form ofcircuit card. The circuit card may include one or more conductors formaking at least one electrical connection between a series resonator onthe first chip and a shunt resonator on the second chip. The circuit maybe a portion of large substrate such that multiple copies of the circuitcard are produced during each repetition of the step 1340. In this case,individual circuit cards may be excised from the substrate and tested aspart of the action at 1340. Alternatively, individual circuit cards maybe excised from the substrate after chips have been attached to thecircuit cards at 1350, or after the devices are packaged at 1360.

At 1350, individual first and second chips are assembled to a circuitcard (which may or may not be a portion of a larger substrate) usingknown processes. For example, the first and second chips may be“flip-chip” mounted to the circuit card using solder or gold bumps orballs to make electrical, mechanical, and thermal connections betweenthe chips and the circuit card. The first and second chips may beassembled to the circuit card in some other manner.

The filter device is completed at 1360. Completing the filter device at1360 includes packaging and testing. Completing the filter device at1360 may include excising individual circuit card/chip assemblies from alarger substrate before or after packaging.

Closing Comments

Throughout this description, the embodiments and examples shown shouldbe considered as exemplars, rather than limitations on the apparatus andprocedures disclosed or claimed. Although many of the examples presentedherein involve specific combinations of method acts or system elements,it should be understood that those acts and those elements may becombined in other ways to accomplish the same objectives. With regard toflowcharts, additional and fewer steps may be taken, and the steps asshown may be combined or further refined to achieve the methodsdescribed herein. Acts, elements and features discussed only inconnection with one embodiment are not intended to be excluded from asimilar role in other embodiments.

As used herein, “plurality” means two or more. As used herein, a “set”of items may include one or more of such items. As used herein, whetherin the written description or the claims, the terms “comprising”,“including”, “carrying”, “having”, “containing”, “involving”, and thelike are to be understood to be open-ended, i.e., to mean including butnot limited to. Only the transitional phrases “consisting of” and“consisting essentially of”, respectively, are closed or semi-closedtransitional phrases with respect to claims. Use of ordinal terms suchas “first”, “second”, “third”, etc., in the claims to modify a claimelement does not by itself connote any priority, precedence, or order ofone claim element over another or the temporal order in which acts of amethod are performed, but are used merely as labels to distinguish oneclaim element having a certain name from another element having a samename (but for use of the ordinal term) to distinguish the claimelements. As used herein, “and/or” means that the listed items arealternatives, but the alternatives also include any combination of thelisted items.

It is claimed:
 1. A 6 GHz Wi-Fi bandpass filter comprising: a ladderfilter circuit comprising two or more shunt transversely-excited filmbulk acoustic resonators (XBARs) and two or more series XBARs, whereineach of the two or more shunt XBARS comprises a diaphragm having anLN-equivalent thickness greater than or equal to 310 nm, and each of thetwo or more series XBARS comprises a diaphragm having an LN-equivalentthickness less than or equal to 305 nm.
 2. The filter of claim 1,wherein each of the two or more shunt XBARS comprises a diaphragm havingan LN-equivalent thickness greater than or equal to 320 nm.
 3. Thefilter of claim 1, wherein each of the two or more series XBARScomprises a diaphragm having an LN-equivalent thickness less than orequal to 295 nm.
 4. The filter of claim 1, wherein the diaphragms of thetwo or more shunt resonators and the two or more series resonatorscomprise a lithium niobate (LN) piezoelectric plate having a thicknessless than or equal to the thickness of the diaphragms of the two or moreseries resonators.
 5. The filter of claim 4, wherein the piezoelectricplate has Euler angles [0°, β, 0° ], where 30°≤β≤38°.
 6. The filter ofclaim 4, wherein the diaphragms of the two or more series resonatorscomprise a dielectric layer with a thickness of tds, and theLN-equivalent thickness of each diaphragm of the at least two seriesresonators is given byteq=tp+ks(tds) where teq is the LN equivalent diaphragm thickness, tp isthe thickness of the LN plate, and ks is a proportionality constant forseries resonators.
 7. The filter of claim 6, wherein the dielectriclayer is silicon dioxide and ks=0.45.
 8. The filter of claim 4, whereinthe diaphragms of the two or more shunt resonators comprise a dielectriclayer with a thickness of tdp, and the LN-equivalent thickness of eachdiaphragm of the two or more shunt resonators is given byteq=tp+kp(tdp) where teq is the LN equivalent diaphragm thickness, tp isthe thickness of the LN plate, and kp is a proportionality constant forshunt resonators.
 9. The filter of claim 8, wherein the dielectric layeris silicon dioxide and kp=0.59.
 10. A 6 GHz Wi-Fi bandpass filtercomprising: a substrate comprising a plurality of cavities; a lithiumniobate (LN) plate supported by the substrate; a plurality ofdiaphragms, each diaphragm comprising a respective portion of thepiezoelectric plate spanning a respective cavity of the plurality ofcavities; and a conductor pattern comprising interdigital transducers(IDTs) of two or more series resonators and two or more shuntresonators, interleaved fingers of each IDT disposed on a respectivediaphragm of the plurality of diaphragms, wherein the diaphragms of thetwo or more shunt XBARS have an LN-equivalent thickness greater than orequal to 310 nm, and the diaphragms of the two or more series XBARS havean LN-equivalent thickness less than or equal to 305 nm.
 11. The filterof claim 10, wherein the conductor pattern further comprises conductorsto connect the two or more series resonators and the two or more shuntresonators in a ladder filter circuit.
 12. The filter of claim 10,wherein the diaphragms of the two or more shunt XBARS have anLN-equivalent thickness greater than or equal to 320 nm.
 13. The filterof claim 10, wherein the diaphragms of the two or more series XBARS havean LN-equivalent thickness less than or equal to 295 nm.
 14. The filterof claim 10, wherein the LN plate has Euler angles [0°, β, 0° ], where30°≤β≤38°.
 15. The filter of claim 4, wherein the diaphragms of the twoor more series resonators comprise a dielectric layer with a thicknessof tds, and the LN-equivalent thickness of each diaphragm of the atleast two series resonators is given byteq=tp+ks(tds) where teq is the LN equivalent diaphragm thickness, tp isthe thickness of the LN plate, and ks is a proportionality constant forseries resonators.
 16. The filter of claim 6, wherein the dielectriclayer is silicon dioxide and ks=0.45.
 17. The filter of claim 4, whereinthe diaphragms of the two or more shunt resonators comprise a dielectriclayer with a thickness of tdp, and the LN-equivalent thickness of eachdiaphragm of the two or more shunt resonators is given byteq=tp+kp(tdp) where teq is the LN equivalent diaphragm thickness, tp isthe thickness of the LN plate, and kp is a proportionality constant forshunt resonators.
 18. The filter of claim 8, wherein the dielectriclayer is silicon dioxide and kp=0.59.